Towards a better hydraulic cleaning strategy for ultrafiltration membrane fouling by humic acid: Effect of backwash water composition

As a routine measurement to alleviate membrane fouling, hydraulic cleaning is of great significance for the steady operation of ultrafiltration（UF） systems in water treatment processes. In this work, a comparative study was performed to investigate the effects of the composition of backwash water on the hydraulic cleaning performance of UF membranes fouled by humic acid（HA）. Various types of backwash water, including UF permeate, Milli-Q water, Na Cl solution, CaCl_2 solution and HA solution, were compared in terms of hydraulically irreversible fouling index, total surface tension and residual HA. The results indicated that Milli-Q water backwash was superior to UF permeate backwash in cleaning HA-fouled membranes, and the backwash water containing Na＋or HA outperformed Milli-Q water in alleviating HA fouling. On the contrary, the presence of Ca^（2＋） in backwash water significantly decreased the backwash efficiency. Moreover, Ca^（2＋） played an important role in foulant removal, and the residual HA content closely related to the residual Ca^（2＋） content.Mechanism analysis suggested that the backwash process may involve fouling layer swelling, ion exchange, electric double layer release and competitive complexation. Ion exchange and competitive complexation played significant roles in the efficient hydraulic cleaning associated with Na＋and HA, respectively.

Fatty acid fouling of forward osmosis membrane:Effects of pH,calcium,membrane orientation,initial permeate flux and foulant composition

Octanoic acid（OA） was selected to represent fatty acids in effluent organic matter（EOM）. The effects of feed solution（FS） properties, membrane orientation and initial permeate flux on OA fouling in forward osmosis（FO） were investigated. The undissociated OA formed a cake layer quickly and caused the water flux to decline significantly in the initial 0.5 hr at unadjusted p H 3.56; while the fully dissociated OA behaved as an anionic surfactant and promoted the water permeation at an elevated p H of 9.00. Moreover, except at the initial stage, the sudden decline of water flux（meaning the occurrence of severe membrane fouling） occurred in two conditions： 1.0.5 mmol/L Ca2＋, active layer facing draw solution（AL-DS） and 1.5 mol/L Na Cl（DS）; 2. No Ca2＋,active layer-facing FS（AL-FS） and 4 mol/L Na Cl（DS）. This demonstrated that cake layer compaction or pore blocking occurred only when enough foulants were absorbed into the membrane surface, and the water permeation was high enough to compact the deposit inside the porous substrate. Furthermore, bovine serum albumin（BSA） was selected as a co-foulant.The water flux of both co-foulants was between the fluxes obtained separately for the two foulants at p H 3.56, and larger than the two values at p H 9.00. This manifested that, at p H 3.56,BSA alleviated the effect of the cake layer caused by OA, and OA enhanced BSA fouling simultaneously; while at p H 9.00, the mutual effects of OA and BSA eased the membrane fouling.